1. Field of the Invention
The present invention relates to a method for forming conversion coatings on metal substrates, such as aluminum or aluminum alloys.
2. Background of the Related Art
Chemical conversion coatings are generally formed by causing the surface of the metal to be “converted” into a tightly adherent coating, all or part of which consists of an oxidized form of the substrate metal. Chemical conversion coatings often provide good corrosion resistance and strong bonding affinity for coatings such as paint. The industrial application of paint to metals generally requires the use of a chemical conversion coating, particularly when the service conditions impose high performance demands.
Although aluminum and aluminum alloys typically offer good corrosion resistance due to the formation of a natural oxide coating at the surface, the protection is limited. Aluminum alloys exposed to a combination of moisture and electrolytes corrode much more rapidly than pure aluminum, especially where such aluminum alloys may contain copper.
There are generally two types of processes for forming corrosion resistant conversion coatings on metal substrates, such as aluminum or aluminum alloy substrates. The first process involves anodic oxidation (anodization) where the substrate is immersed in a chemical bath, such as a chromic or sulfuric acid bath, and an electric current is passed through the substrate and the chemical bath. The conversion coating thus formed on the surface of the substrate provides improved corrosion resistance and an improved bonding surface for organic coatings and finishes.
The second process for forming a corrosion resistant chemical conversion coating produces a chemical conversion coating by subjecting the substrate to a chemical solution, such as a chromic acid solution, but without using an electric current in the process. The chemical solution may be applied through immersion of the substrate, manual application or spray application. The resulting conversion coating on the surface of the aluminum or aluminum alloy substrate provides improved resistance to corrosion and an improved bonding surface for organic coatings and finishes.
Chromate based conversion coatings have been widely used in applications where maximum corrosion protection is needed. For example, treating aluminum or aluminum alloy substrates with a chromate conversion coating bath generally results in a favorably thick, corrosion resistant film consisting of hydrated Cr (III) and Al (III) oxides. This reaction is driven by the reduction of high-valent Cr (VI) ions and the oxidation of the Al metal. The benefits of this chromate conversion coating include hydrophobicity and self-healing properties.
The light weight and high strength of aluminum and aluminum alloys make these materials particularly useful in aviation and aerospace applications. Many aluminum structural parts, including Cd-plated aluminum, Zn-plated aluminum and Zn—Ni plated aluminum, are currently being treated using chromic acid process technology. Chromic acid conversion films, as formed on aluminum and aluminum alloy substrates, meet the ASTM Method B-117 168-hour salt fog exposure corrosion resistance criterion, but they primarily serve as a substrate surface for coatings or paint adhesion. Chromic acid conversion coatings are relatively thin and low in weight coatings (40-150 milligrams per square foot), and do not cause unfavorable reductions in the fatigue life of the aluminum and aluminum alloy structures to which they are applied.
The use of chromate conversion coatings for aluminum and aluminum alloy substrates, as well as other substrates, are not without drawbacks. Researchers have increasingly found problems with chromate conversion coatings related to their extreme toxicity and carcinogenocity. Researchers have linked exposure to chromates to a variety of human illnesses including irritation of the respiratory tract, ulcerations and perforations of the nasal septum, dermatitis, skin sensitization, asthma and lung cancer. As a result of these findings, federal and state environmental regulations have been promulgated, particularly in California, as well as in other countries, that impose drastic restrictions on the allowable levels of hexavalent chromium (Cr (IV)) compounds in effluents and emissions related to metal finishing processes. Consequently, chemical conversion processes employing hexavalent chromium compounds have become prohibitively expensive, if permissible at all, and this has given rise to the need for an alternative means of achieving comparable material properties without the use of chromates.
Recent efforts to produce non-chromate conversion coatings have involved the use of other oxidizing agents including cerium compounds, alkaline solutions of lithium salts, and manganates and molybdates. Investigators have studied the effects of cerium compounds as a corrosion inhibitor for aluminum and copper alloys such as Al 2024-T3 in chloride-containing solutions. It was proposed that cerium inhibits corrosion of this alloy by reducing the rate of cathodic reduction of oxygen due to formation of cerium (III)-rich films over copper containing intermetallics that act as local cathodic sites.
A process for surface modification of aluminum-based materials that involves immersion in boiling cerium salts followed by anodic polarization in a molybdate solution has been reported. Although this surface modification process produced good corrosion resistant films, the long-term boiling of the substrate presented problems of pre-treating large structures. The problems of long-term boiling along with those of the electrochemical post-treatment step made this process unattractive for practical applications.
An unusual passivity of aluminum alloys has been found when the aluminum alloys are exposed to alkaline solutions of lithium salts. The observed passivity has been explained as a consequence of the formation of a polycrystalline Li2[Al2(OH)6]2CO3.3H2O film on the aluminum alloy surface. This film, referred to as hydrotalcite or “talc” coating, has been reported to offer increased corrosion protection during exposure to aggressive environments. The best results, however, were obtained when the coated samples were allowed to cure for at least one week before any corrosion test was made. This extremely long cure time would undoubtedly cause problems in practical industrial applications of talc coatings. Although talc coatings improve the corrosion resistance of various substrates, only alloys with low concentrations of alloying elements (Al 6061-T6 and Al 1100) passed the ASTM Method B-117 salt fog test.
Attention has also been directed towards the use of manganates and molybdates in conversion coating solutions for aluminum alloys. The permanganate conversion coating solutions included salts, such as silicates, borates, nitrates, halides and phosphates.
Isomolybdates were shown to improve the corrosion resistance of aluminum and aluminum alloys against localized attack by shifting the breakdown potential (Eb) in a positive direction. The following reactions are believed to be involved in the formation of a molybdenum-based conversion coating on aluminum:MoO42−+5H++Al=Mo3++½Al2O3.3H2O+H2O3MoO42−+6H++2Al=3MoO2+Al2O3.3H2O
The treatment converts the aluminum surface to a superficial layer containing a complex mixture of aluminum/molybdenum compounds. It has been shown that the hydrated Mo4+ concentration in the film at all potentials was approximately 2 to 3 times greater than the concentration of the hexavalent Mo6+. It has been suggested that the corrosion resistance of these molybdate coatings was due to the molybdate (VI)-rich regions on the film surface that inhibited the ingress of Cl− anions to the metal/film interface. In the presence of alkaline solutions, however, molybdenum has a slight tendency to decompose water with the evolution of hydrogen, dissolving the molybdate in the hexavalent state as the molybdate ion, MoO42−, thus weakening the conversion coating on the metal surface. Thus, to prepare a suitable hexavalent molybdate (Mo6+) conversion solution, it will be necessary to operate in an alkaline condition with a pH greater than 10. In molybdate-free solutions at pH 10, AlOOH that would naturally form under lower pH conditions is not suitable and will tend to dissolve. The presence of molybdates in the solutions is not sufficient to limit the rapid dissolution of the Al and, hence, formation of a conversion coating based on isomolybdates under these conditions is unfavorable.
Therefore, there is a need for a conversion coating solution containing non-toxic ions that form a stable corrosion resistant conversion coating on metal surfaces, particularly on aluminum and aluminum alloys. It is desirable that the conversion coating solution be suitable for sound adherence of an applied protective coating, such as paint. There is also a need for a method for using a conversion coating solution containing non-toxic ions to form a stable corrosion resistant conversion coating on metal surfaces, particularly on aluminum and aluminum alloys.